1. Field of the Invention
The present invention relates to an optical recording medium, and an optical recording medium recording/reproducing method and apparatus for recording or reproducing digital data on and from the optical recording medium.
2. Description of the Related Background Art
A crosstalk canceling technique is known as a method of recording and reproducing digital data on and from an optical disc as an optical recording medium at a high density. This technique is applied to playing back an optical disc on which data is recorded at a high density with a narrower track pitch, for canceling crosstalk introduced into a central track from neighboring tracks on both inner and outer sides thereof by irradiating the three neighboring tracks with three reading laser light beams and adding and/or subtracting read signals produced by the respective laser light beams for reading.
FIG. 1 illustrates an optical disc recording/reproducing apparatus to which the crosstalk canceling technique is applied. On a recording surface of an optical disc 1, recording points are arranged in a lattice form. FIG. 1 also illustrates by enlarging a portion A on the recording surface of the optical disc 1, where black dots represent recording points at which binary digital data is respectively recorded in the form of presence or absence of a pit.
A recording head 2 turns on or off a recording laser light beam in accordance with information data x to form a pit train on a track of the disc 1. Specifically, the recording head 2 turns on the recording laser light beam to form a pit at a recording point when information data is "1," and turns off the recording laser light beam to not form a pit at a recording point when information data is "0."
A reading head 3 irradiates three neighboring tracks on the optical disc 1 with three reading laser light beams B1, B2, B3, respectively, and opto-electrically transduces the respective reading laser light beams after they have been reflected from the optical disc 1 to produce three read signals p1, p2, p3 in accordance with trains of pits recorded on the three tracks. In this case, since the tracks are formed at a narrow track pitch, the read signal p2 from the central track suffers crosstalk introduced from the read signals p1, p3 from the tracks on both inner and outer sides thereof.
A crosstalk canceling circuit 4 receives the three read signals p1, p2, p3, and adds and/or subtracts these signals in a predetermined ratio to cancel crosstalk. Specifically, the read signals p1, p3 from the neighboring tracks are multiplied by predetermined coefficients to duplicate crosstalk introduced into the read signal p2 from the central track. Then, the crosstalk thus duplicated is subtracted from the read signal p2 on the central track to cancel the crosstalk therefrom.
A decoder 5 determines the level of a read signal after the crosstalk has been eliminated therefrom to reconstruct digital data as reproduced data z. The reproduced data z reconstructed in this way is equal to recording data y2 on the central track. In other words, the relationship between these data z and y2 is expressed by: EQU z=y2 (1)
With the use of the crosstalk canceling technique as described above, the track pitch can be narrowed to some degree to enhance recording density of the optical disc. However, the reduction of the track pitch is substantially limited to the radius of a reading laser light beam. When the track pitch is made significantly narrower than the radius of the beam, the crosstalk canceling circuit can no longer eliminate crosstalk because the narrower track pitch causes an increased amount of crosstalk. Since it is impossible to significantly reduce the track pitch as compared with the radius of the reading laser light beam in the conventional recording/reproducing method, there is a limit to the enhancement of the recording density on the optical disc.